(1) Field of the Invention
The present invention lies in the field of methods of deicing outside surfaces of aircraft. More specifically, the present invention relates to ways of deicing the blades of a rotorcraft rotor.
(2) Description of Related Art
In the field of aviation, the problem arises of outside surfaces of aircraft becoming iced. An aircraft may potentially fly under icing conditions that lead to the formation of rime or even solid ice on its outside surfaces, which is not desirable. Rime or ice forming on the outside surfaces of an aircraft leads in particular to the aircraft becoming heavier and also affects its aerodynamic characteristics, thereby affecting its performance.
Icing conditions for the outside surfaces of an aircraft are typically identified in application of various criteria including in particular, for an aircraft or given structure, the overall structure of the aircraft and the arrangement of its outside surfaces under consideration, the aircraft's operating point within its flight envelope, and atmospheric conditions.
The atmospheric conditions identifying icing conditions for outside surfaces of an aircraft under consideration conventionally vary depending on various meteorological parameters of values that are identified during flight tests.
Among such meteorological parameters, account is taken in particular of ambient temperature, of the concentration of water in the ambient outside air, and of the mean volume diameter of the water droplets contained in the ambient outside air.
For a rotorcraft in particular, it is observed more particularly that the blades of the rotors of the aircraft lose the greatest amount of performance when the aircraft is flying under icing conditions in a determined ambient temperature range that is referred to below as the “critical temperature domain”.
The critical temperature domain is identified in particular by temperature sensors, and it extends between a “low” temperature and a “high” temperature. The concentration of liquid water in the ambient outside air is commonly identified by icing detectors and/or by specific probes, for example.
The mean volume diameter of droplets of water contained in the ambient outside air is identified in particular by laser droplet size probes, such as forward scattering spectrometer probes (FFSP) or cloud droplet probes (CDP).
Other criteria based on the perception of the pilot may also potentially be taken into account for identifying icing conditions on outside surfaces of an aircraft, such as a variation in the flying behavior of the aircraft, and in particular in its vibratory behavior.
As a result of measurements taken during said test flights, the pilot of a production aircraft having the same structure as the aircraft used during said test flights can identify the icing conditions to which the outside surfaces of the production aircraft are subjected and can consequently adapt accordingly the way in which the production aircraft is flown.
The pilot of the production aircraft can in particular identify said icing conditions on the basis of values for meteorological parameters as supplied by the on-board instrumentation of the aircraft and/or by a weather station, and on the basis of the pilot's own experience of the impact of meteorological conditions on the behavior of the production aircraft.
In addition, means for deicing the outside surfaces of an aircraft are commonly operated as a result of icing conditions being identified, either by the human pilot or else by ice detectors identifying the presence of ice on the outside surfaces of the aircraft, or indeed by instruments of the kind described in Document GB 2 046 690 (Secretary of State for Defense). Such instruments serve in particular to identify the presence of ice on the blades of a rotorcraft rotor by comparing the real driving torque delivered to the rotor with a predetermined driving torque corresponding to operation under normal meteorological conditions, i.e. in the absence of icing. By way of example, the predetermined driving torque may be calculated in particular as a function of the collective pitch (angle) of the blades of the rotorcraft rotor.
In this context, and more particularly concerning rotorcraft flying under icing conditions, the formation of rime or ice on the blades of a rotorcraft rotor affects the performance of the rotor.
The formation of rime or ice on the blades of a rotorcraft rotor is particularly harmful for the main rotor of a rotorcraft that serves essentially to provide the rotorcraft with lift, and possibly also with propulsion and/or guidance in flight in the specific example of a helicopter.
When a rotorcraft is flying under icing conditions, any rime or ice picked up on the blades of the rotor(s) of the rotorcraft greatly decreases rotor performance. Such a loss of rotor performance is induced in particular by a rapid increase in the drag of the blades because of the deposition of rime or ice changing their aerodynamic profile.
Furthermore, the rotors of a rotorcraft are conventionally driven in rotation by a power plant of the rotorcraft that includes at least one fuel-burning engine, in particular a turboshaft engine.
A control unit generates a setpoint for the speed at which each rotor, and in particular the main rotor, should be driven, which setpoint is referred to as the NR setpoint. The NR setpoint is transmitted to a regulator unit that regulates the speed of operation of the engine(s) in order to drive the rotor(s), and in particular the main rotor, at a speed referred to as the NR speed, that complies with the NR setpoint.
Proposals have also been made concerning a main rotor of a rotorcraft to cause the value of the NR setpoint to vary over a range of values extending by way of example approximately from 92% to 107% of the nominal speed of rotation at which the main rotor is to be driven. Variation in the value of the NR setpoint is controlled by the control unit in application of various criteria for achieving specific results, such as a reduction in the noise generated by the main rotor during a stage of approaching a landing point, or increasing the performance of the rotorcraft when flying in various specific stages of flight.
In this context, reference may be made for example to the following Documents: FR 3 000 465 (Airbus Helicopter); US 2007/118254 (G. W. Barnes et al.); and WO 2010/143051 (Agusta S P A et al.), which disclose various ways of controlling such variation in the NR speed of a rotorcraft main rotor.
In a field remote from the present invention and relating to wind turbines for producing electricity, methods are known for identifying icing conditions of the blades of a rotor of a turbine as a function of meteorological conditions.
By way of example, reference may be made on this topic to the following Documents: EP 1 936 186 (General Electric company); US 2012/0226485 (A. Creagh et al.); and EP 2 626 557 (Siemens A G).
More particularly with reference to Document EP 2 626 557, proposals have also been made to limit the icing of the blades of a wind turbine rotor by varying the speed of rotation of said rotor. More particularly, information about the rate of heat loss from the blades is collected and the speed of the rotor is controlled depending on that information.
By way of example, the rate of heat loss from the blades is determined on the basis of a simulation or of a physical model of the blades while the rotor is exposed to a given ambient temperature and for a given ambient wind characterized by its speed relative to the ground.
Still with reference to EP 2 626 557, account may also be taken of the speed of rotation of the wind turbine, the rate of heat loss from the blades possibly increasing with a decrease in ambient temperature, depending on the increase in wind speed and/or depending on the increase in the speed of rotation of the turbine. The speed of rotation of the turbine is controlled with reference to the rate of heat loss from the blades, for example by adjusting the electrical power generated by the turbine.